1. Field of the Invention
The present invention relates to a communication device, a communication method, a program, and a storage medium. More particularly, the present invention relates to a communication device, a communication method, a program, and a storage medium storing a program, which allow multiple access in UWB (ultra wideband) communication.
2. Description of the Related Art
UWB communication is also called impulse radio communication and is expected to find various applications, as discussed for example, in xe2x80x9cImpulse radio: How it worksxe2x80x9d by M. Z. Win and R. A. Scholtz (IEEE Communication Letters, Vol. 2, pp. 36-38, February, 1998 (hereinafter, this literature will be denoted as xe2x80x9cRef. 1xe2x80x9d)), xe2x80x9cUWB waveforms ad coding for communications and radarxe2x80x9d by L Fullerton (Proc. IEEE Telesystems Conf., pp. 139-141, Mar. 26-27, 1991 (hereinafter, this literature will be denoted as xe2x80x9cRef. 2xe2x80x9d)), and xe2x80x9cAnswers to questions posed by Bob Lucky, Chairman of the FCC""s Technical Advisory Committee (e-mail correspondence, http://ultra.usc.edu./ulab, Jun. 29, 1999 (hereinafter, this literature will be denoted as xe2x80x9cRef. 3xe2x80x9d)).
In UWB communication, data is transmitted using a very short pulse at low power without using a carrier. To transmit such a very short pulse, a wide bandwidth on the order of several GHz is used, as described, for example, in Ref. 1 or 2.
The UWB communication technology has many advantages that stem from its ultra-wideband nature, as described in, for example, xe2x80x9cOn the robustness of ultra-wideband width signals in dense multipath environmentsxe2x80x9d by M. Z. Win and R. A. Scholtz (IEEE Communication Letters, Vol. 2, pp. 51-53, February, 1998 (hereinafter, this literature will be denoted as xe2x80x9cRef. 4xe2x80x9d)), and xe2x80x9cOn the analysis of UWB communication channelsxe2x80x9d by J. M. Cramer, R. A. Scholtz, and M. Z. Win (Proc. IEEE Military Communications Conf., pp. 1191-1195, Oct. 31 to Nov. 3, 1999 (hereinafter, denoted as xe2x80x9cRef. 5xe2x80x9d)). More specifically, it can penetrate walls, experience significantly less fading, offer extremely fine time-resolution, and deliver large processing gains.
In particular, the absence of a carrier signal in the UWB communication obviates the need for radio frequency (RF) or intermediate frequency (IF) circuits. This allows reductions in device size and power consumption.
These characteristics of the UWB communication technology give it substantial advantages over conventional narrowband, wideband and infrared wireless communication systems.
Potential applications of the UWB communication include wireless local area networks (LAN), information distribution systems for use in a medical application or the like, outdoor or indoor wireless multiple-access communication systems, ranging devices, and other many applications, as described in, for example, xe2x80x9cCatching the wave; Breakthroughs in wireless technologyxe2x80x9d by J. S. Gage (MD Computing: The Leading Edge in Medical and Healthcare Informatics, Vol. 16, March/April 1999 (hereinafter, denoted as xe2x80x9cRef. 6xe2x80x9d)), and xe2x80x9cUltra-wideband width time-hopping spread-spectrum impulse radio for wireless multiple-access communicationsxe2x80x9d by M. Z. Win and R. A. Scholtz (IEEE Trans. Communications, Vol. 48, pp. 679-691, April, 2000 (hereinafter, denoted as xe2x80x9cRef. 7xe2x80x9d)).
There is a great need for applying the UWB communication technology to multiple-access (multi-user) communication with a plurality of communication terminals. The idea of applying UWB to multiple-access communication is discussed in, for example, xe2x80x9cMultiple access with time-hopping impulse modulationxe2x80x9d by R. A. Scholtz (Proc. IEEE Military Communications Conf. (Boston, U.S.), pp. 447-450, Oct. 11-14, 1993 (hereinafter, denoted as xe2x80x9cRef. 8xe2x80x9d)), and xe2x80x9cMultiple-access performance limits with time hopping and pulse position modulationxe2x80x9d by F. Ramirez-Mireles and R. A. Scholtz (Proc. IEEE Military Communications Conf., pp. 529-533, Oct. 18-21, 1998 (hereinafter, denoted as xe2x80x9cRef. 9xe2x80x9d)).
UWB multiple-access communication systems proposed in Refs. 7 to 9 are conceptually similar to asynchronous code-division multiple-access (CDMA) systems in certain aspects. Transmitted signals of each user (signals transmitted from each communication terminal) share a common spectrum, and the set of signaling waveforms across all users are not necessarily orthogonal.
The multiple-access capability of a UWB system is primarily determined by the processing gain that is given by the pulse bandwidth to the pulse (or symbol) repetition frequency (RPF) ratio. In the UWB communication, its very wide frequency bandwidth results in a high processing gain. The UWB communication technology is very different from the CDMA communication technology in that the high processing gain of the UWB communication allows it to be advantageously employed in applications in which power is limited.
In the literatures Ref. 1 and Refs. 7 to 9, it is proposed to realize multiple-access UWB communication by time-hopping the timing of transmitting pulse-position-modulated signals of respective users on the basis of a time-hopping sequence indicated by a pseudorandom number.
Herein, the xe2x80x9cpulse position modulationxe2x80x9d refers to modulation of pulse positions depending on data to be transmitted. For example, a unit period with a predetermined length is assigned to one symbol, and each period is divided into M intervals. A pulse is placed in one of the M intervals, at a position corresponding to data to be transmitted. The xe2x80x9ctime hoppingxe2x80x9d refers to an operation of, rather than transmitting symbols (unit periods) at fixed intervals, randomly shifting the transmission timing from the fixed intervals.
In the proposed UWB multiple-access communication techniques, each receiving device includes a single-user demodulator for detecting the pulse position, in each unit period, of a pulse-position-modulated signal.
The single-user demodulator includes a filter bank including filters for matching the received signal with pulses at possible positions in unit periods, and a detector for detecting a filter that provides a highest output among the filters of the filter bank, and employing, as demodulated data, a value corresponding to a pulse position that is matched with the received signal by the filter that is determined to provide the highest output.
Ideally, in the multiple-access communication, there is no overlap among pulses of signals transmitted from users. Under such an ideal condition, the single-user demodulator can function as an ideal demodulator for demodulating a pulse-position-modulated signal in an environment including additive white Gaussian noise. In this case, the single-user demodulator may be constructed in a similar fashion to a single-user matched filter for detection in CDMA systems, as described, for example, in xe2x80x9cMultiuser detection for CDMA systemsxe2x80x9d by A. Duel-Hallen, J. Holtzman and Z. Zvonar (IEEE Personal Communications Magazine, Vol. 2, pp. 46-58, April, 1995 (hereinafter, denoted as xe2x80x9cRef. 10xe2x80x9d)), xe2x80x9cMulti-user detection for DS-CDMA communicationsxe2x80x9d, by S. Moshavi (IEEE Communications Magazine, Vol. 34, pp. 124-136, October, 1996 (hereinafter, denoted as xe2x80x9cRef. 11xe2x80x9d)), and xe2x80x9cMultiuser Detectionxe2x80x9d by S. Verdu (Cambridge University Press, 1998 (hereinafter, denoted as xe2x80x9cRef. 12xe2x80x9d)).
However, in practical multiple-access communication systems, interference occurs among signals transmitted from users (hereinafter, such interference will be referred to as multiple-access interference), and multiple-access interference (MAI) is generally not Gaussian. The conventional single-user modulator is no longer optimal in such an environment in which MAI can occur.
MAI increases with the number of multiple-access users, and the increase in MAI causes various adverse effects on the single-user demodulator. When data is transmitted at a high rate, the resultant high pulse repetition frequency causes a reduction in the processing gain and thus an increase in MAI.
In view of the above, it is an object of the present invention to provide a high-performance demodulator for use in UWB multiple-access communication.
According to an aspect of the present invention, there is provided a communication device comprising correlation calculation means for calculating the correlation between a received signal corresponding to signals transmitted from a plurality of communication terminals by means of UWB (ultra wideband) communication and pulses at possible positions in a signal transmitted from each communication terminal; and demodulation means for demodulating data transmitted from the respective communication terminals on the basis of the correlation, taking into account interference among the signals transmitted from the communication terminals.
According to another aspect of the present invention, there is provided a communication method comprising the steps of calculating the correlation between a received signal corresponding to signals transmitted from a plurality of communication terminals by means of UWB (ultra wideband) communication and pulses at possible positions in a signal transmitted from each communication terminal; and demodulating data transmitted from the respective communication terminals on the basis of the correlation, taking into account interference among the signals transmitted from the communication terminals.
According to still another aspect of the present invention, there is provided a program comprising the steps of calculating the correlation between a received signal corresponding to signals transmitted from a plurality of communication terminals by means of UWB (ultra wideband) communication and pulses at possible positions in a signal transmitted from each communication terminal; and demodulating data transmitted from the respective communication terminals on the basis of the correlation, taking into account interference among the signals transmitted from the communication terminals.
According to still another aspect of the present invention, there is provided a storage medium storing program comprising the steps of calculating the correlation between a received signal corresponding to signals transmitted from a plurality of communication terminals by means of UWB (ultra wideband) communication and pulses at possible positions in a signal transmitted from each communication terminal; and demodulating data transmitted from the respective communication terminals on the basis of the correlation, taking into account interference among the signals transmitted from the communication terminals.
That is, in the communication device, the communication method, and the program according to the present invention, the correlation is calculated between the received signal corresponding to the signals transmitted from a plurality of communication terminals by means of UWB (Ultra WideBand) communication technology and pulses at possible positions in the transmitted signals from the respective communication terminals, and the received signal is demodulated into original data issued by the communication terminals on the basis of the calculated correlation taking into account interference among the transmitted signals from the respective communication terminals.